In my opinion, "fast love" does not exist. It could refer to the "chemistry" or attraction to the other person that could happen the first time you meet that person. This attraction would have to grow and develop into a deeper feeling, as you get to know the person, appreciate that person for his/her attributes, and learn to understand that person, if it is going to be true love. This process takes time to really get to know the person. A relationship could grow into everlasting love that could last a life-time.
There is something called "infatuation" that is a quick attraction to someone, but this often dies just as quickly as it starts. A person should let their relationship grow slowly if it is to be a long-lasting love relationship. Become good friends first!